In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
One approach to achieving nm-scale feature sizes in semiconductor devices is the use of short wavelengths of light, for example, 193 nm or less, during exposure of chemically amplified photoresists. Immersion lithography effectively increases the numerical aperture of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer. The immersion fluid allows a greater amount of light to be focused into the resist layer than would occur with an air or inert gas medium.
The theoretical resolution limit as defined by the Rayleigh equation is shown below:
  R  =            k      1        ⁢          λ      NA      where k1 is the process factor, λ is the wavelength of the imaging tool and NA is the numerical aperture of the imaging lens. When using water as the immersion fluid, the maximum numerical aperture can be increased, for example, from 1.2 to 1.35. For a k1 of 0.25 in the case of printing line and space patterns, 193 nm immersion scanners would only be capable of resolving 36 nm half-pitch line and space patterns. The resolution for printing contact holes or arbitrary 2D patterns is further limited due to the low aerial image contrast with a dark field mask wherein the theoretical limit for k1 is 0.35. The smallest half-pitch of contact holes is thus limited to about 50 nm. The standard immersion lithography process is generally not suitable for manufacture of devices requiring greater resolution.
Considerable effort has been made to extend the practical resolution capabilities of positive tone development in immersion lithography from both a materials and processing standpoint. One such example involves negative tone development (NTD) of a traditionally positive-type chemically amplified photoresist. NTD is an image reversal technique allowing for use of the superior imaging quality obtained with bright field masks for printing the critical dark field layers. NTD resists typically employ a resin having acid-labile (or acid-cleavable) groups and a photoacid generator. Exposure to actinic radiation causes the photoacid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups, giving rise to a polarity switch in the exposed regions. As a result, a difference in solubility characteristics is created between exposed and unexposed regions of the resist such that unexposed regions of the resist can be removed by particular developers, typically organic developers such as ketones, esters or ethers, leaving behind a pattern created by the insoluble exposed regions. Such a process is described, for example, in U.S. Pat. No. 6,790,579, to Goodall et al. That document discloses a photoresist composition comprising an acid-generating initiator and a polycyclic polymer containing recurring acid labile pendant groups along the polymer backbone. The exposed areas can be selectively removed with an alkaline developer or, alternatively, the unexposed regions can be selectively removed by treatment with a suitable nonpolar solvent for negative tone development.
Certain problems can result when applying conventional 193 nm photoresists to the NTD process. The developed photoresist pattern can, for example, demonstrate significant thickness loss as compared with the pre-exposed resist layer. This can give rise to pattern defects resulting from complete erosion of portions of the resist pattern during subsequent etching. Thickness loss is believed to be caused by cleavage and loss of commonly employed bulky acid labile groups such as large tertiary alkyl ester groups from the resist layer. Thickness loss for conventional 193 nm photoresists which rely solely on such bulky acid labile groups for polarity switching can be particularly problematic due to the high content of such groups. The use of a thicker resist layer may not be a practical solution as other issues such as reduction in the depth of focus and pattern collapse can then result. The occurrence of pattern collapse when using typical 193 nm photoresists for NTD is believed to be exacerbated by the relatively high content of (meth)acrylic acid units generated in exposed regions of the photoresist following cleavage of certain acid-labile groups from (meth)acrylate-based polymers especially where such groups are solely responsible for the polarity switch. The (meth)acrylic acid units contribute to poor adhesion on organic and Si-based inorganic substrates due to the polarity mismatch between resist patterns and substrates. Another problem associated with the use in NTD of such conventional photoresists relying solely on the aforementioned bulky acid labile groups for polarity switching is etch resistance reduction.
There is a continuing need in the art for improved polymers, photoresist compositions and photolithographic methods for negative tone development which allow for the formation of fine patterns in electronic device fabrication and which avoid or conspicuously ameliorate one or more of the foregoing problems associated with the state of the art.